Tilt-dependent beam-shape system

ABSTRACT

The present invention relates to a system for changing the radiation pattern shape of an antenna array during electrical tilting. The antenna array has multiple antenna elements, and the system comprises a phase-shifting device provided with a primary port configured to receive a transmit signal, and multiple secondary ports configured to provide phase shifted output signals to each antenna element. The system further comprises a phase-taper device that changes phase taper over the antenna elements, and thus the beam shape, with tilt angle θ. The invention is adapted for use in down-link as well as up-link within a wireless communication system.

This application is the U.S. national phase of International ApplicationNo. PCT/SE2006/001170, filed 16 Oct. 2006, the entire contents of whichis hereby incorporated by reference.

TECHNICAL FIELD

The technology disclosed herein relates to a system for adapting thebeam-shape of an antenna in a wireless communication network.

BACKGROUND

Variable beam tilt is an important tool for optimizing radio accessnetworks for cellular telephony and data communications. By varying themain beam pointing direction of the base station antenna, bothinterference environment and cell coverage area can be controlled.

Variable electrical beam tilt is conventionally performed by adding avariable linear phase shift to the excitation of the antenna elements,or groups of elements, by means of some phase-shifting device. For costreasons, this phase-shifting device should be as simple and contain asfew components as possible. It is therefore often realized using somekinds of variable delay lines. In the description, the terms “linear”and “non-linear” should be understood to refer to relative phase overmultiple secondary ports of a multiport phase shifting network, and notthe time or phase behaviour of a port in itself.

Conventional multi-port phase shifters, with one primary port and anumber N (N>1) secondary ports, are implemented with linear progressivevariable phase taper over the secondary ports. In addition to the linearprogressive phase taper, fixed amplitude and phase tapers are often usedas a means for generating a tapered nominal secondary port distribution.

FIGS. 1 a and 1 b illustrate a conventional phase shifter 10, with oneprimary port 11, and the phase shifter generates in down-link linearprogressive phase shifts over four secondary ports 12 ₁-12 ₄. Avariable-angle “delay board” 13 has multiple trombone lines 14, one foreach secondary port 12 ₁-12 ₄. The trombones lines 14 are arranged atlinearly progressive radii. By a proper choice of junctionconfigurations, line lengths, and line impedance values, the nominalphase and amplitude taper of the phase shifter can be controlled, forexample to achieve uniform phase over the secondary ports as indicatedby “0” in FIG. 1 a. By changing the delay line lengths (i.e. the lengthof the trombone lines 14), in this case by rotation of the delay board13 relative to a fixed board 15, the secondary ports 12 ₁-12 ₄experience linear progressive phase shifts as indicated in FIG. 1 b. Inup-link, the secondary ports 12 ₁-12 ₄ receive signals from an antenna(not shown) which are combined within the phase shifter to a commonreceive signal at the primary port 11.

The use of non-linear phase-shifting devices for controlling electricaldown tilt has been contemplated, such as mentioned in U.S. Pat. No.5,798,675, by Drach, U.S. Pat. No. 5,801,600, by Butland et al.

A system for tilt-dependent beam shaping using conventional linear phaseshifters is disclosed in JP 2004 229220. The system has different beamwidth depending on the tilt angle, but this is achieved by a tilt anglecontrol section (41) in combination with a vertical beam width controlsection (42) in the base station controller (4), see FIG. 6 in JP 2004229220.

Traditionally, base station antennas have had a variable beam tilt rangeof approximately one beamwidth. This together with the fact that mostmobile connections today are circuit switched voice with a fixedrequirement on bit-rates, has not triggered any interest in improvingthe Signal-to-interference+noise ratio (SINR) close to the antenna.Normally it is good enough.

For particular cell configurations, e.g. highly placed antennas incombination with small cells, the need for using antennas with largebeam tilt is greater. For antennas with conventional narrow elevationbeam radiation patterns, the large beam tilt causes users close to thebase station to experience a lower path gain than users close to thecell border, since the difference in path loss for the near and farusers is smaller than the difference in directive antenna gain. Forpacket-based data communication this is not optimal usage of theavailable power. Therefore, for antennas with large beam tilt, somedegree of radiation pattern null-fill below the main beam, or even somecosec-like beam-shaping is desirable.

In large cells, on the other hand, when no or small beam tilt isemployed, the antenna pattern should be optimized for maximum peak gain.The path gain for the users at the cell border will anyway be smallerthan for users closer to the base station because the path loss variesrapidly with vertical observation angle in the case of large cells andobservation angles close to the horizon.

SUMMARY

The technology disclosed herein provides a system that allows aradiation pattern of an antenna to be optimized both for high maximumgain at small tilt angles, and high degree of null filling below themain beam at large tilt angles.

The technology disclosed herein provides a system for changing the beamshape of an antenna, preferably having multiple antenna elementsarranged in an array, in dependency of a tilt angle. Electric tilting isachieved by including a phase-shifting device that will provide phaseshifts over secondary ports from the phase-shifting device. Aphase-taper device provides changed phase taper over the antennaelements with tilt angle.

An advantage with the technology disclosed herein is that a singleantenna may be used in an adaptive system, to fulfil the need forincreasing the quality of a communication link and thus increase the bitrate associated with one or more simultaneous users, by maintaining anoptimal antenna pattern, which depends on the distance to the basestation.

Further objects and advantages will become apparent for a skilled personfrom the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b show a linear phase shifter.

FIGS. 2 a and 2 b show a first embodiment of a non-linear phase shifter.

FIGS. 3 a and 3 b show diagrams illustrating phase shifts from thelinear and non-linear phase shifters.

FIG. 4 shows a second embodiment of a non-linear phase shifter.

FIG. 5 shows antenna element excitation at 0° beam tilt.

FIG. 6 shows antenna element excitation at 9° beam tilt.

FIGS. 7 a-7 d show elevation radiation patterns utilizing the technologydisclosed herein.

FIG. 8 shows a wireless telecommunication network having base stationsincluding the technology disclosed herein.

FIG. 9 schematically illustrates the tilt dependent beam shape accordingto the technology disclosed herein.

DETAILED DESCRIPTION

A base station, including an antenna with multiple antenna elements, isarranged within a cell, where the characteristics of the antennadetermine the size of the cell and the cell coverage area all else beingequal. To accomplish the same signal strength in the entire cell,independent of the distance to the base station, the antenna gain G(θ)divided by the path loss L(θ) should be constant in the cell, as afunction of observation angle θ:

$\frac{G(\theta)}{L(\theta)} = {C = {{const}.}}$

However, the constant C changes with cell configuration, i.e. antennainstallation height and cell size, which in turn means that the optimalantenna radiation pattern changes with beam tilt angle, as illustratedin FIGS. 7 b-7 d, lines 71. The tilt dependent radiation pattern can beaccomplished by changing the phase taper over the antenna withtilt-angle, e.g. by providing a non-linear phase shifter as described inconnection with FIGS. 2 a, 2 b, 3 b and 4. The non-linear phase shifterfacilitates different phase tapers for different beam tilt angles, andthus will provide tilt-dependent beam shape of the antenna.

The terms “phase shift” and “time delay” are used interchangeably in thefollowing description and it should be understood that these terms referto equivalent properties in the present context, except if otherwisenoted.

An essential part of the technology disclosed herein is to providenon-linear phase taper over the secondary ports of a phase shifternetwork. A method for achieving this is to use a multi-secondary porttrue time delay network in which the relative delay line lengths are, ingeneral, non-linearly progressive. A true time delay network generatesfrequency-dependent phase shifts, a property which makes it particularlysuitable for antenna applications, such as beam-steering.

The principle idea of a first embodiment of a non-linear phase shifter20, in down-link, is illustrated in FIGS. 2 a and 2 b using a true timedelay network, similar to the one illustrated in FIGS. 1 a and 1 b. Thekey property of the delay network (and the method as such) is to providenon-linear relative time delays over the secondary ports, by arrangingtrombone lines 24 (in this particular embodiment) in a non-periodicfashion on a delay board 23. By a proper choice of junctionconfigurations, line lengths, and line impedance values, the nominalphase and amplitude taper of the true time delay network with non-lineardelay dependence can be controlled, for example to achieve uniform phaseover the secondary ports as indicated by “0” at the secondary ports 12₁-12 ₄ in FIG. 2 a. In contrast with the true time delay network in FIG.1, changes in the delay line lengths by rotation of the delay boardrelative to a fixed board 25 produces non-linear progressive time delays(and, hence, phase shifts) over the secondary ports 12 ₁-12 ₄, asindicated by “φ₁”, “φ₂”, “φ₃”, and “φ₄” in FIG. 2 b. In up-link, thesecondary ports 12 ₁-12 ₄ of the phase shifter 20 receive signals froman antenna (not shown) which are non-linearly time-delayed and combinedwithin the phase shifter to a common receive signal at the primary port11.

As a non-limiting example, the phase-shifts from a linear and anon-linear true time delay network in down-link are compared in FIGS. 3a and 3 b for different rotations (see legend) of the delay board 13 and23, respectively. In FIG. 3 a, the phase advance (relative phase) overthe secondary ports 12 ₁-12 ₄ is linear with delay board 13 rotation,which manifests itself as straight lines 30, 31, 32 and 33 for a givenboard rotation. This means that for any given delay board rotation, therelative phase values (between secondary port n and port 1) areΔφ_(n)=(n−1)Δφ=(n−1)kα,where n is the secondary port number, α is the board rotation angle, andk is a constant that depends on implementation aspects, for example wavenumber of transmission lines and radial separation of the trombones 14.

The non-linear phase advance (relative phase) over the secondary ports12 ₁-12 ₄ of a non-linear true time delay network is illustrated in FIG.3 b. In FIG. 3 b, the phase advance (relative phase) over the secondaryports 12 ₁-12 ₄ is non-linear when rotating the delay board 23, whichmanifests itself as one straight line 35 for 0° rotation and threenon-straight lines 36, 37 and 38 for a given board rotation ≠0°. Thus,the relative phase values are not identical, i.e.,φ_(n)−φ_(n−1)≠φ_(n+1)−φ_(n), for at least one n, nε{2,N−1}wherein N is the number of delay branches. In FIG. 3 b, the phase ofdelay branch 3 varies faster than twice that of branch 2 when the boardangle changes.

FIG. 4 shows a second embodiment of a non-linear phase shifter 40. Thisdelay line network is based on translation (rather than rotation) of thedelay board 43 relative a fixed board 45. The delay network trombonelines 44 are shown with equal lengths, but they could also havedifferent lengths (both the lines on the delay board 43 and the lines onthe fixed board 45).

FIG. 5 shows an element excitation of a 15 element linear antenna array,optimized for maximum gain and a suppression of the upper sidelobes to−20 dB. This element excitation produces the radiation pattern in FIG. 7a, i.e. 0° beam tilt. In prior art techniques, linearly progressivephase is added to the phase taper shown in FIG. 5 to achieve differenttilt angles, θ_(tilt).

FIG. 6 shows the element excitation for 9.degree. beam tilt, where theamplitude taper is the same as for 0.degree. beam tilt, but the phasetaper has been optimized for null-filling, in accordance with thetechnology disclosed herein. This excitation produces the radiationpattern with 9° beam tilt in FIG. 7 d.

For beam tilt angles between 0° and 9°, the phase excitation is found bya linear interpolation of the phase excitations at 0° and 9°. Some ofthese radiation patterns 70 are shown in FIGS. 7 b and 7 c, with thebeam tilt changing 3° for each subplot. For comparison, the relativepath loss 71, normalized at beam peak, is shown in the same plots. Therelative path loss changes with beam tilt angle θ_(tilt).

The technology disclosed herein is not limited to the example withconstant cell illumination described above, but is applicable in allcases where it is desirable, for one reason or another, to have aradiation pattern that changes with beam tilt angle. Furthermore, thetechnology disclosed herein is not limited to linear antenna arrays, butmay also be implemented in a base station having a non-linear antennaarray.

The technology disclosed herein allows the antenna pattern to beoptimized both for high maximum gain at small tilt angles, and for goodcoverage (high degree of null filling) close to the antenna at largetilt angles θ_(tilt).

FIG. 8 shows a wireless telecommunication system 80, exemplified usingGSM standard, including a first base station BS₁. The first base stationBS₁ is connected via a first base station controller BSC₁ to a corenetwork 81 of the telecommunication system 80. A uniform linear antennaarray 83 comprises in this embodiment six antenna elements 84. Secondaryports 12 of a non-linear phase shifter 85 is connected to each antennaelement 84 of the uniform linear antenna array 83, and a primary port 11of the phase shifter 85 is connected to the first base station BS₁. Thefirst base station controller BSC₁ controls the variable beam tilt bychanging the position of a non-linear delay board, as previouslydescribed in connection with FIGS. 2 a, 2 b and 4, and thereby alteringthe beam shape of a beam from the uniform linear antenna array 83.

The telecommunication system 80 also includes a second base station BS₂.The second base station BS₂ is connected via a second base stationcontroller BSC₂ to the core network 81. A non-uniform linear antennaarray 88 comprises in this embodiment four antenna elements 84, notnecessarily cross polarized as illustrated. Secondary ports 12 of alinear phase shifter 10 (prior art) are connected, via a phase-taperdevice 87 that changes the phase taper over the antenna elements withtilt angle θ_(tilt), to each antenna elements 84 of the non-linearantenna array 88. A primary port 11 of the phase shifter 10 is connectedto the second base station BS₂. The second base station controller BSC₂controls the variable beam tilt by changing the position of a lineardelay board, as previously described in connection with FIGS. 1 a and 1b, and thereby altering the beam shape of a beam from the non-uniformlinear antenna array 88.

It should be noted that the antenna array may have uniformly, ornon-uniformly, arranged antenna elements 84, and cross polarized antennaelements are only shown as a non-limiting example and other types ofantenna elements may naturally be used without deviating from the scopeof the invention. Furthermore, antenna elements operating in differentfrequency bands may be interleaved without departing from the scope ofthe claims.

The illustrated telecommunication system (GSM) should be considered as anon-limiting example, and other wireless telecommunication standards,such as WCDMA, WiMAX, WiBro, CDMA2000, etc. may implement the describedtechnology disclosed herein without deviating from the scope of thetechnology disclosed herein. Some of the described parts of the GSMsystem, e.g. base station controller BSC_(i) and BSC₂ may be omitted incertain telecommunication standards, which is obvious for a skilledperson in the art.

FIG. 9 illustrates an antenna array 83 arranged in an elevated position,such as in a mast 90. A non-linear phase shifter 85 is connected to theantenna array 83 (as described in connection with FIG. 8) and iscontrolled by a base station controller BSC₁. A non-tilted beam 91(corresponding to the 0° plot in FIG. 7 a) is illustrated in FIG. 9together with a tilted beam 92 (corresponding to the 9° plot in FIG. 7d).

Although the technology disclosed herein has been described in detailusing down-link, the skilled person in the art may readily adapt theteachings for up-link, as is mentioned above.

1. A system for changing the radiation pattern shape of an antenna arrayin down-link during electrical tilting, said antenna array comprisingmultiple antenna elements, said system comprising: a phase-shiftingdevice provided with a primary port configured to receive a transmitsignal, and multiple secondary ports configured to provide phase shiftedoutput signals to each antenna element; a phase-taper device thatconfigured to change phase taper over the antenna elements, and thus thebeam shape, with tilt angle (θ_(tilt)), wherein said phase-taper deviceis integrated with said phase-shifting device, to form a non-linearphase-shifting device; and wherein the phase-shifting device comprises adelay line network with trombone lines, and said non-linearphase-shifting device generates non-linear progressive phase shifts overthe secondary ports when changing tilt angle (θ_(tilt)).
 2. A system forchanging the radiation pattern shape of an antenna array in up-linkduring electrical tilting, said antenna array comprising multipleantenna elements, said system comprising: a phase-shifting deviceprovided with multiple of secondary ports configured to receive phaseshifted input signals from each antenna element, and a primary portconfigured to combine the input signals to a receive signal; aphase-taper device that configured to change phase taper over thesecondary ports, and thus the beam shape, with tilt angle (θ_(tilt)),wherein said phase-taper device is integrated with said phase-shiftingdevice, to form a non-linear phase-shifting device; and wherein thephase-shifting device comprises a delay line network with trombone linesand said non-linear phase-shifting device generates non-linearprogressive phase shifts over the secondary ports when changing tiltangle (θ_(tilt)).
 3. The system according to claim 1 or 2, wherein thesame phase-shifting device is used for down-link and up-link.
 4. Thesystem according to claim 3, wherein said phase-shifting devicecomprises a movable member which provides said non-linear progressivephase shifts.
 5. The system according to claim 4, wherein said movablemember has a rotational movement.
 6. The system according to claim 4,wherein said movable member has a translational movement.
 7. The systemaccording to claim 3 or 4, wherein the system is configured tocommunicate phase shifted signals to/from antenna elements arranged in auniform antenna array.
 8. The system according to claim 1 or 2, whereinthe system is configured to communicate phase shifted signals to/fromantenna elements arranged in a non-uniform antenna array.
 9. A methodfor changing the radiation pattern shape of an antenna array indown-link during electrical tilting, said antenna array having multipleantenna elements, said method comprising: providing phase shifted outputsignals to each antenna element from multiple secondary ports of a phaseshifting device, said phase-shifting device is provided with a primaryport configured to receive a transmit signal; providing changed phasetaper over the antenna elements with tilt angle (θ_(tilt)) using aphase-taper device, wherein said method further comprises integratingsaid phase-taper device with said phase-shifting device, to form anon-linear phase-shifting device; wherein said method further comprisesgenerating non-linear progressive phase shifts over the secondary portsof the non-linear phase-shifting device with tilt angle (θ_(tilt)); andwherein the act of generating non-linear progressive phase shifts isimplement as a delay line network with trombone lines.
 10. A method forchanging the radiation pattern shape of an antenna array in up-linkduring electrical tilting, said antenna array having multiple antennaelements, said method comprising: providing phase shifted input signalsfrom each antenna element to multiple secondary ports of a phaseshifting device, said phase-shifting device is provided with a primaryport configured to combine the input signals to a receive signal;providing changed phase taper over the secondary ports with tilt angle(θ_(tilt)) using a phase-taper device, wherein said method furthercomprises integrating said phase-taper device with said phase-shiftingdevice, to form a non-linear phase-shifting device; wherein said methodfurther comprises generating non-linear progressive phase shifts overthe secondary ports of the non-linear phase-shifting device with tiltangle (θ_(tilt));and wherein the act of generating non-linearprogressive phase shifts is implement as a delay line network withtrombone lines.
 11. The method according to claim 9 or 10, comprisingthe step of using the same phase-shifting device for down-link andup-link.
 12. The method according to claim 11, wherein the act ofgenerating non-linear progressive phase shift is performed by moving amovable member.
 13. The method according to claim 12, wherein movingsaid movable member includes a rotational movement.
 14. The methodaccording to claim 12, wherein moving said movable member includes atranslational movement.
 15. The method according to claim 11 or 12,wherein the method comprises the additional step of configuring thesystem to communicate phase shifted signals to/from antenna elementsarranged in a uniform antenna array.
 16. The method according to claim 9or 10, wherein the method comprises the additional step of configuringthe system to communicate phase shifted signals to/from antenna elementsarranged in a non-uniform antenna array.
 17. A base station adapted tobe used in a communication network in down-link, said base stationcomprising: an antenna array comprising multiple antenna elements; aphase shifting device provided with: a primary port configured toreceive a transmit signal, and multiple secondary ports configured toprovide phase shifted output signals to each antenna element; said phaseshifting device being configured to be controlled by a controller toperform electrical tilt of a beam; a phase-taper device that changesphase taper over the antenna elements, and thus the beam shape, withtilt angle (θ_(tilt)), wherein said phase-taper device is integratedwith said phase-shifting device, to form a non-linear phase-shiftingdevice; and wherein the phase-shifting device comprises a delay linenetwork with trombone lines and said non-linear phase-shifting devicegenerates non-linear progressive phase shifts over the secondary portswhen changing tilt angle (θ_(tilt)).
 18. A base station adapted to beused in a communication network in up-link, said base stationcomprising: an antenna array comprising multiple antenna elements; aphase shifting device provided with: multiple secondary ports configuredto receive phase shifted input signals from each antenna element; and aprimary port configured to combine the received input signals to areceive signal; said phase shifting device being configured to becontrolled by a controller to perform electrical tilt of a beam; aphase-taper device that changes phase taper over the secondary ports,and thus the beam shape, with tilt angle (θ_(tilt)), wherein saidphase-taper device is integrated with said phase-shifting device, toform a non-linear phase-shifting device; and wherein the phase-shiftingdevice comprises a delay line network with trombone lines and saidnon-linear phase-shifting device generates non-linear progressive phaseshifts over the secondary ports when changing tilt angle (θ_(tilt)). 19.The base station according to claim 17 or 18, wherein the samephase-shifting device is used for down-link and up-link.
 20. The basestation according to claim 17 or 18, wherein the base station comprisesa uniform antenna array.
 21. The base station according to claim 19 or20, wherein said base station comprises a non-uniform antenna array. 22.A communication network comprising at least one base station accordingto claim 17 or 18.